Method of making insulating bodies



Aug. 13, 1968 F METHOD OF MAKING INSULATING BODIES 4 Sheets-Sheet 1SWEET W/TH CALOTTE SHAPED PROJECT/0N8 Original Filed March 15, 1962 F/Gi/a T w 5 W A H SHEET WITH PROJECT/0N8 WITH TR/ANGULAR BASE V%% A A AVAiv/WV FIG. lb

SHEET W/TH PROJECT/0N3 WITH OVAL BASE PLAIN SHEET FIG. /c

T H H S W m. P

INVENTOR. flea; Feta? Ria /f f Aug. 13, 1968" H. FELIX METHOD OF MAKINGINSULATING BODIES Original Filed March 15, 1962 4 Sheets-Sheet 2INVENTOR. h'd -J' Fafz} Aug. 13, 1968 H. FELIX METHOD OF MAKINGINSULATING BODIES 4 Sheets-Sheet 5 Original Filed March 15. 1962INVENTOR. du P: Cir

Aug. 13, 1968 FEUX METHOD OF MAKING INSULATING BODIES Original FiledMarch 15. 1962 4 Sheets-Sheet 4 FIG. 7

CREPE PLAIN PAPER INTERMEDIATE LAYER LAYER PLA/IV INTERMEDIATE LAYERINVENTQR. /au: Fedx United States Patent 3,397,098 METHOD OF MAKINGINSULATING BODIES Hans Felix, Olten, Switzerland, assignor toMaser-Glaser Co. A.G., Muttenz, near Basel, Switzerland Originalapplication Mar. 15, 1962, Ser. No. 179,946, now Patent No. 3,250,850,dated May 10, 1966. Divided and this application Feb. 14, 1964, Ser. No.344,988 Claims priority, application Switzerland, Feb. 17, 1961,1,977/61; Mar. 16, 1961, 3,160/61 5 Claims. (Cl. 156-48) ABSTRACT OF THEDISCLOSURE A unitary, insulated portion of an electrical apparatus isformed by arranging about an elongated electrically conductive member,coaxially therewith, a plurality of superposed continuous first layersof insulating material and of continuous second layers alternating withthe first layers, the first layers having a plurality of spacedprojections which extend toward a contact adjacent second layers so thatadjacent convolutions of the second layers are completely separated fromeach other by intervening convolutions of the first layers andintermeshing of adjacent convolutions of the first layers is preventedand, due to the spaced projections of the first layers, the interveningspaces between adjacent convolutions of the second layers, and the spaceadjacent the conductive member, are divided into a plurality ofpassage-s. The thus-formed structure is then subjected to a partialvacuum in order to be dried and degassed, and thereafter the passagesare filled With a fiowable hardenable casting resin while the partialvacuum is still maintained, followed by hardening of the casting resinwhich fills the pasasges.

The present invention relates to laminated coverings and to a method ofmaking the same, more particularly, the present invention is concernedwith insulated electrically conductive bodies such as condenser bushingsand other portions of electrical apparatus.

The present application is a division of my copending application Ser.No. 179,946, filed Mar. 15, 1962, entitled Insulating Body and Method ofMaking the Same, now Patent No. 3,250,850, which application Ser. No.179,946, filed Mar. 15, 1962, in turn, is a continuationin-part ofapplication Ser. No. 174,133, filed Feb. 19, 1962, entitled InsulatedElectrically Conductive Bodies and Method of Making the Same, nowabandoned.

It has been proposed to apply laminated coverings to electricalapparatus by forming a laminated body about such electrical apparatusconsisting of smooth papers such as kraft paper layers which werecombined by means of synthetic resins so as to form a laminated body.Condensation resins and solvent-free impregnating and adhesive resinssuch as epoxy resins and unsaturated polyester or thermoplastic resins,or resins broadly described as casting resins were primarily used foradhering the individual smooth paper layers to each other.

However, it was not possible in this manner to assure the formation ofcrack-free laminated coverings.

It was also suggested to replace the smooth paper layers with corrugatedsheet material. The term corrugated will be used throughout the presentspecification and claims in its broad meaning as defined in the secondedition of Websters Unabridged New International Dictionary. However,the intermeshing of adjacent cor-rugated sheet layers tends to causedifiiculties and, furthermore, the fact that corrugated sheetssubstantially lend themselves to bending only in a direction parallel tothat of the corrugations, requires the use of relatively narrow stripsof such material when curved pieces of electrical 3,397,098 PatentedAug. 13, 1968 apparatus or other curved solid members are to be providedwith such laminated covering.

It is therefore an object of the present invention to provide animproved laminated covering and a method of making the same, thelaminated covering to serve as an insulating body.

Other objects and advantages of the present invention will becomeapparent from a further reading of the description and of the appendedclaims.

With the above and other objects in view, the present inventioncontemplates in an electrical apparatus, a laminated body havinginsulating properties and comprising, in combination, a plurality ofsuperposed first layers of insulating material and second layersalternating with the first layers, the first layers having oppositefaces and being formed respectively, with a plurality of spacedprojecting hollow deformations extending outwardly from at least one ofthe faces toward an adjacent one of the plurality of second layers so asto form a free space between adjacent layers, the projecting hollowdeformations of the first layers, respectively, being arranged in such amanner that each of the first layers possesses a reduced resistance tobending in at least one direction, and a cast resin filling the freespaces between adjacent ones of the plurality of superposed layers.

The present invention also proposes in an electrical apparatus, alaminated body having insulating properties and comprising, incombination, a plurality of superposed layers of insulating materialhaving opposite faces, respectively, the layers, respectively, beingformed with a plurality of spaced projecting hollow deformationsextending outwardly from at least one of the faces toward an adjacentone of the plurality of layers so as to form a free space betweenadjacent layers, the projecting hollow deformations of the layers,respectively, being arranged in such a manner that each of the layerspossesses a reduced resistance to bending in more than one direction,and a cast resin filling the free spaces between adjacent ones of theplurality of superposed insulating layers.

According to one embodiment, the present invention provides in anelectrical apparatus, a laminated body having insulating properties andcomprising, in combination, a plurality of superposed first layers ofelastically tensioned crepe paper, a plurality of second plain sheetlayers alternating with the first layers arranged so as to form freespaces between adjacent layers, and hardened synthetic resin filling thespaces between adjacent layers so as to form a crack-free laminatedbody.

According to a preferred embodiment, a method of forming a rigidcrack-free laminated covering on an elongated substantially cylindricalsolid body of predetermined diameter is proposed which comprises thesteps of forming a plurality of tubes of pretensioned corrugated sheetmaterial the corrugations of which extend substantially parallel to theaxes of the tubes, the tubes being of progressively increasing diametersso that the tubes may be inserted into each other in spacedrelationship, the diameter of the smallest tube in tensioned state beingsufli- -cient to permit insertion into the same of the substantiallycylindrical solid body, forming a plurality of second tubes of plainsheet material of progressively increasing diameters such that the tubesof plain sheet material may be inserted between adjacent ones of thecorrugated tubes, assembling the tubes about the cylindrical solid bodyunder tensioning of the first tubes so as to form a compositecylindrical structure formed of the cylindrical solid body surrounded byalternatingly arranged tubular layers of tensioned corrugated and ofplain sheet material, impregnating the thus formed composite structurewith a synthetic hardenable resin so as to fill all voids therein, andhardening the resin whereby due to the tensioned state of the corrugatedtubes the same will be able to conform to any dimensional changes causedby the hardening of the resin, thereby forming a rigid crack-freelaminated covering on the elongated solid body.

Thus, the electrical apparatus of the present invention may comprise anelongated solid electrical conductor having a curved axis, and alaminated body having insulating properties at least partially coveringthe conductor, the laminated body comprising a plurality of superposedfirst layers of insulating material and second layers alternating withthe first layers, respectively, each of the superposed layers beingformed of a spirally wound relatively narrow band of sheet materialhaving opposite faces and being arranged substantially coaxially withthe solid electrical conductor, the layers being formed with a pluralityof spaced projections arranged in two rows extending substantially atright angles to each other, the projections extending outwardly from atleast one of the faces toward an adjacent one of the plurality of layersso as to form a free space between adjacent layers, the projections ofthe layers, respectively, being arranged in such a manner that thelayers possess a minimum resistance to bending in more than onedirection, and a cast resin filling the free spaces between adjacentones of the plurality of superposed insulating layers.

The present invention is also concerned with a method of producing anelectrical apparatus comprising the steps of arranging about anelongated electrically conductive member coaxially therewith a pluralityof superposed first layers of insulating material and second layersalternating with the first layers, the first layers, respectively beingformed with a plurality of spaced projections extending toward andcontacting adjacent ones of the second layers so as to form a free spacebetween the layers and adjacent the conductive member, filling the freespace with a hardenable casting resin in flowable state, and allowingthe casting resin filling the space to harden, whereby a unitary,insulated portion of an electrical apparatus is formed.

The carrier layers according to the present invention may be helicallywound in the form of relatively narrow bands so as to form concentriclayers. In this manner, i.e. by using bands which will form concentriclayers with a plurality of spaced projections, the above discusseddifiiculties will be overcome. It is important that the material ofwhich the layers are to be formed has such projections extendingoutwardly in at least one direction from the plane of the material, andthat the projections are so shaped and arranged that it is possible tobend the material in more than one direction without irreversibledeformation of the material of which the insulating layer is formed,will possess a minimum resistance to bending in more than one, i.e., inseveral directions.

It should be avoided that the projections of adjacent layers interlock.For this reason, as will be described in more detail below, preferably aplain layer, i.e. a layer without projections, is interposed betweenadjacent layers formed with the above described spaced projections.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments, when read in connection with the accompanying drawings, inwhich:

FIG. 1 (a-c) illustrates plan views of fragments of band shapedmaterials of which the insulating layers according to the presentinvention may be formed;

FIG. 2 is a perspective cross sectional view of an insulatedelectrically conductive member according to the present invention;

FIG. 3 (a-c) illustrates axial and transversal cross sections through aninsulated electrically conductive member according to the presentinvention, partly before and partly after introduction of the castingresin;

FIG. 4 is a schematic elevational illustration of the winding of theinsulating layer about an elongated electrically conductive core member;

FIG. 5 is an elevational, fragmentary cross sectional view through aportion of the projections carrying layer adhered to adjacent plainlayers, and

FIGS. 6 and 7 illustrate corrugated papers or sheets which may be usedaccording to the present invention, such as the coarse crepe paper shownin FIG. 6 or the highly tensionable crepe or pleated paper sheet of FIG.7;

FIG. 8 is a schematic illustration of a manner in which a crack-freelaminated covering may be formed on an electrically conductive member;and

FIG. 9 is an exploded somewhat schematic view of a multi-layer tubularbody produced in accordance with the present invention.

Referring now to the drawings, and particularly to FIG. 1, it will beseen that the projections or pimples which preferably are embossed intothe carrier layer material, may be of different shape such ascalotte-shaped, part-spherical, semi-spherical, triangular, etc. Theprojections may extend outwardly from one face of the sheet material oralternatingly from one and the other face, so that on each faceprojections and indentations alternate, as shown in FIGS. la, 2, 3a, 3b,3c and 5.

Such embossed sheet material with so-called honeycomb embossing orsemi-spherical embossing are produced for instance by the firmBaurngartner in Lausanne, Switzerland. Furthermore, other types of sheetmaterial, produced by the above named finm as decorative paper under thetrade names Carbion B and Carbion C may also be used for the purposes ofthe present invention.

As illustrated and stated above, the shape of the projections or pimplesneed not be limited to the shape of a calotte or the like. FIG. 1b showsprojections having a triagonal base, and it would be also possible tomake projections having a square or rhornboid base, usually with more orless rounded corners. Oval or elliptic projections also give goodresults. Carrier sheets embossed with a combination of differentlyshaped projections may be used, or carrier sheets which carry acontinuous, for instance, zigzag pattern of alternating projections andindentations. However, as has already been stated, it is also possibleto use as material for the carrier layer a sheet which has been embossedin such a manner as to have allll projections extend outwardly from oneface of the s eet.

Referring now to the perspective cross sectional view of FIG. 2, whereinthe left-hand portion of the figure shows a transversal cross sectionand the right-hand portion of the figure a longitudinal cross section,it will be seen that the high tension conductor 1 is surrounded byseveral carrier layers 2. The coaxially arranged carrier layers 2 serveas reinforcements of the cast resin and for the spacing of condenserlayers 3. As illustrated, the carrier layers are formed withsemi-spherical projections which alternatingly extend outwardly from oneand the other face of the respective carrier layer. Thus, between thecarrier layers a system of free spaces is formed which communicate witheach other in axial and tangential direction and which are then filledwith a casting resin, preferably with a low casting resin indicated byreference numeral 4.

FIG. 3a shows in axial direction the carrier layer skeleton and thecondenser layers between the individual carrier layers, prior to theintroduction of the casting resin. It is shown that the view through thecarrier layer skeleton is blocked by the semi-spherical projectionswhich extend transversely throughout the entire carrier layer skeleton.Thus, viewed in axial direction, it would appear as if the skeleton wereblocked by spherical or pearlshaped elements.

The cross sectional view FIG. 30 is taken after introduction andhardening of the casting resin 4. The cross sectional views of FIG. 3aand 3c are taken in a direction perpendicular to the axis of electricconductor 1, while FIG. 3b is a cross sectional view taken inlongitudinal direction of the insulated electric conductor. Here again,reference numeral 1 denotes the electric conductor, reference numeral 4the casting resin and, depending on whether the cross sectional planecuts through the projections or between the same, the carrier layerswill be seen as indicated by reference numeral 2 or 2'. The condenserlayers are again identified by reference numeral 3.

According to FIG. 4, the band of projections and indentations 5 and 6carrying material 2 is helically wound about conductor 1. This isrepeated so that several coaxial layers 2 with interposed condenserlayer 3 will be superposed upon each other. The last formed layer isindicated by reference numeral 4. The width of band 2 is preferably sochosen that the entire lateral edges of the band are located eitherbetween the projections and indentations or cut the same in half.

The casting resin which is used to fill the interspaces between layers 2and 3 may contain, in a manner known per se, filler materials,particularly pulverulent inert filler materials such as quartz powder orsand.

It is a particular advantage of the present invention that there will bevery little separation of the pulverulent filler during hardening of theresin so that in the finished article the pulverulent filler materialwill be substantially evenly distributed throughout the hardened castingresin.

It is possible and sometimes desirable to insert between the individuallayers of projections-carrying sheets plane, i.e. projection-free layersof absorbent or porous material. Or, as illustrated in FIG. 5, acomposite structure may be wound about the conductive core, namely aprojection carrying layer adhered at both of its faces, i.e. at theapices of the projections, to plain cover layers or sheets. The coverlayers as well as the carrier layers 2 may be formed with perforationsin order to facilitate passage of flowable casting resin and completefilling of the free spaces within the carrier layer skeleton with thesame.

The cover layers may also consist of electrically conductive material,for instance, of metal foil, or stressgrading layers, or condensorlayers may be separately arranged between the insulating carrier layers.

Depending on the composition and quality of the carrier layer material,the cover layers may be adhered to the carrier layers in variousmanners, adhesively or by plastic welding, and such cover layers may bearranged on one or both faces of the respective projections-carryingcarrier layer.

Due to the fact that accoring to the present invention the carriermaterial may be wound helically in the form of relatively narrow bandsto form concentric layers, it-

is now possible to produce very long or curved condenser bushings.Furthermore, when forming straight condenser bushings, it is no longernecessary to carefully cut rectangular carrier layer sheets to veryclose tolerances.

Furthermore, the use of a carrier material which is formed at least atone face thereof with spaced relief-like projections, will result in abetter statistical distribution, i.e. in a more even distribution of thecarrier layer material in the interior of the insulating casting resinbody. In additon, the shape and arrangement of the projections willresult in a more advantageous distribution of the casting resin relativeto the carrier layer skeleton. As compared with prior art structures inwhich the carrier layer skeleton is formed of corrugated cardboard,insulating bodies produced according to the present invention will be ofa more homogenous or isotropic structure. In other words, there will bea more even distribution of carrier layer material, casting resin andfiller material within the casting resin throughout the entirestructure. For instance, according to the present invention, straightchannels of even cross section and similarly shaped casting resin rodsfilling the same, will be avoided and this will substantially eliminatethe danger of transversal crack formation.

The further disadvantage of unhomogenous distribution of mineral fillerssuch as quartz powder due to gradual separation and settling of thefiller during the hardening of the resin, is substantially prevented byforming carrier layers as described herein. Experiments have shown thatsuch carrier mate-rials according to the present invention, i.e. theconfiguration of the spaces which are then filled with casting resin,will have a retarding effect on the separation and settling of thefiller material. Thus, according to the present invention, it ispossible to introduce highly fluid casting resins and the greater thefluidity of the casting resins the bigger and longer may be theinsulating bodies formed in this matter. Up to now, it was verydiflicult to operate with highly fluid casting resins due to thesettling of pulverulent filler materials therein. This difficulty isovercome to a very large extent by the present invention and thus it ispossible with the carrier skeleton arrangement disclosed herein toproduce larger insulating bodies with filler material-containing castingresins.

Preferably, the embossed carrier layers according to the presentinvention are produced from an absorbent porous sheet material such as afibrous sheet material, particularly cellulose fibers containingmaterial, a paper or paper-like products.

The projections which are arranged on at least one face of the carrierlayer are so arranged that the layer material possesses very littleresistance to bending in more than one direction. For instance,inspection of FIG. la will show that there will be little resistance tobending in horizontal direction and in direction perpendicular anddiagonal thereto. Thus, winding of the material about a core can becarried out without mechanical destruction thereof, i.e. without tearingof the embossed sheet material or band and without subjecting the sameto undue mechanical stresses.

Due to the extension of the projections perpendicular to the plane ofthe sheet, the maximum transverse or perpendicular dimension of theembossed sheet will be a multiple of the thickness of the initial sheet.

As shown in FIG. 9, the rigid crack-free laminated covering on anelongated substantially cylindrical body 91 is produced by alternatinglysuperimposing on body 91 first tubes 92 and second tubes 93' ofprogressively increasing diameters so that said tubes are inserted intoeach other in spaced relationship. First tubes 92 consist of corrugatedsheet material with corrugations extending substantially parallel to theaxes of the tubes, and second tubes 93 being formed of plain sheetmaterial and are interposed be tween adjacent first tubes. Afterassembling the tubes, the formed composite structure is impregnated witha synthetic hardenable resin so as to fill the voids of the structure,and the resin is then hardened.

The following examples are given as illustrative only without limitingthe invention to the specific data of the examples.

Example 1.Straight cylindrical condenser bushing Six layers of a carriermaterial such as is illustrated in FIG. 1a are arranged on a straightcopper rod of circular cross section having a diameter of 30 mm. and alength of 1400 mm. The total thickness of the six layers equals 18 mm.Between adjacent layers a perforated electrically conductive insertof-Hochstatter paper, ie.. a paper supported aluminum foil, is arranged.The length of the conductive insert is reduced in radial direction as itis customary in the case of condensor bushings. The carrier layersupporting the respective conductive insert exceeds the length of theconductive insert exceeds the length of the conductive insert by about 5mm. on each side.

The carrier material is cut into strips having a width of mm. and iswound about the conductive copper rod in helical manner so that theedges of adjacent windings contact each other.

The conductive inserts are placed coaxially onto the underlying woundcarrier layer in the form of rectangular sheets, and are fixed to thecarrier layer with adhesive tape. After forming the first layer in theabove described manner, five additional layers are similarly superposed.Left-hand and right-hand windings will alternate between adjacentsuperposed layers.

The copper rod with the carrier layers and conductive inserts fixedthereto is then inserted coaxially into a castin'g mold of 84 mm. innerdiameter. The mold is closed at its lower end and provide-d with afeeding tunnel at its upper end. The mold is installed in uprightposition in a 'heatable vacuum chamber and the contents of the mold aredried and degassed for four hours at a pressure of 0.5 mm.

Thereafter, while the vacuum is maintained, the casting resin isintroduced through the funnel at a temperature of 80 C. After 10minutes, the vacuum is released and the resin is then hardened for 12hours at 90 C. and thereafter for 8 hours at 120 C. After cooling, thethus formed body is removed from the mold and a terminal is affixed tothe outermost stress grading layer for later grounding.

The materials used in the present example are, for the carrier layer, anembossed, machine-smooth cellulose paper having a structure asillustrated in FIG. 1a, a total thickness of 3 mm. and an unembossedsheet thickness of 0.5 mm. The weight of the unembossed paper equals 375grams per square meter.

The casting resin mixture consists of 100 parts by weight of epoxy resinon the basis of p,p -dioxydiphenylpropane, having an epoxy equivalenceweight of between 185 and 200, and a viscosity of between 11,000 and14,000 centipoises at 25 C.; 130 parts by weight of dodecenylsuccinicacid anhydride; 0.2 part by weight of methylbenzyldimethyl-amine; and300 parts by weight of quartz powder (325 mesh per inchaccording toTyler).

Resin and acid anhydride are mixed well and then dried. The quartzpowder is added and as last component the methylbenzyldimethylamine. Thecomponents were then intimately mixed for 10 minutes.

Example 2.-Curved condenser bushing Six coaxial layers of the carrierand spacing material are formed about a circular copper rod of 22 mm.diameter and 1600 mm. length, which rod had been bent at its center toan angle of 60, with a radius of 500 mm. Between the individual carrierlayers step-graded electrically conductive inserts are arranged. Thecarrier material is helically wound in the form of bands having a widthof 60 mm. about the straight portions of the copper rod, while thewindings about the curved portion of the copper rod are formed of bandshaving a width of 15 mm. The windings about the curved portion of thecopper rod are so formed that the bent edges at the inner curvature ofthe rod contact each other while there will be a free space of a fewmillimeters between adjacent band edges at the outer curvature of therod. About each thus formed carrier layer, a smooth paper band of 15 mm.width is helically wound, and coated for the desired distance with agraphite lacquer. Then follows the next layer of carrier material andthereon the next helically wound layer of partially graphite lacqueredsmooth paper. The total thickness of the six layers equals 19 mm.

The thus covered copper rod is installed in a metal mold of 67 mm. innerdiameter in such a manner that the distance between the covered rod andthe inner mold wall remains even throughout. The lower end of the moldis closed and a funnel is arranged at its upper end. The mold isinstalled in as upright position as possible in a vacuum oven, dried anddegassed for hours at 80 C. and 0.5 mm. Hg. Thereafter, the castingmixture is introduced through the funnel at a temperature of 80 C. whilevacuum is maintained. After 15 minutes atmospheric pressure is restoredand the resin is hardened for 8 12 hours at C. and thereafter for 8hours at 120 C.

The bands of carrier material consist of embossed cellulose paper asillustrated in FIG. 1a, having a total thickness of 2.8 mm. and anunembossed thickness of 0.25 mm. The paper weight equals 200 grams persquare meter.

The smooth paper which carries the graphite laquer is a kraft paperhaving a thickness of 0.15 mm. and a weight of 113 grams per squaremeter. The composition of the casting mixture is the same as in Example1.

Example 3.Cylindrical straight condenser bushing The arangement,dimensions and the quality of the carrier material are the same as inExample 1. However, the casting mixture consists of parts by weight ofRhodester 1108R a solution of an unsaturated polyester in styrene madeby the firm Rhone-Poulec, Paris, France, and having a viscosity of 300centipoises at 25 C.; one part by weight benzoyl peroxide; and 150 partsby weight quartz powder (325 mesh per inch).

Resin hardener and well dried filler are intimately mixed and cast at 40C.

Hardening is carried out for 8 hours at 80 C. and for 12 hours at C.

When corrugated sheet material such as crepe paper or the like is to beused, preferably in pretensioned state, to form part of the laminatedcovering, it has been found, according to the present invention, thatsuperior results are achieved when intermeshing or interlocking ofadjacent crepe paper layers is avoided, so that the pretensionedindividual crepe paper layers may expand or contract withoutinterference from adjacent crepe paper layers. The pretensioned crepepaper or the like layer preferably is in a condition wherein furthertensioning would lead to further expansion and, on the other hand,reduction in the applied tension would lead to contraction of the layer.This is important in order to permit the corrugated, pretension layer toadjust to dimensional changes occurring during hardening of thehardenable or casting resin used for impregnating the composite layerstructure. Such resin, F. 1, may contract somewhat during hardeningthereof. For this purpose, according to the present invention, smooth orplain sheet layers are interposed between adjacent corrugated sheetlayers.

By proceeding in this manner, namely so that the individual pretensionedcrepe paper layers may contract or expand, independent of and withoutinterference by adjacent superposed crepe paper layers, it is possibleto substantially eliminate any difi'erences which may exist in thedegree of pretensioning of the crepe paper layers. In other words, byproceeding as outlined above, the degree of pretensioning of all of thesuperposed crepe paper layers can be equalized.

When it is intended to build up the laminated body of tubular crepepaper or the like layers alternating with plain tubular sheet layers,then the fact that the individual tubular crepe paper layers are movablerelative to the adjacent plain sheet layers, will permit to give to eachof the tubular crepe paper layers a desired degree of pretensioning,which will be equal throughout the entire respective tubular crepe paperlayer. This ability of the corr-ugated or crepe paper layers to contrastor expand so as to adjust to any dimensional changes of the syntheticresin which may occur during hardening of the latter, will facilitatethe formation of crack-free laminated bodies. It is well known and neednot be discussed in detail herein that the absence of cracks or voids ininsulating bodies covering portions of electrical apparatus, such ascondenser bushings, is a very important requirement.

Thus, according to the present invention, it is proposed to interposeintermediate layers of smooth, plain, foiltype sheet material betweenadjacent superposed, preferably pretensioned, layers of corrugated sheetmaterial such as crepe paper. The characteristics and thickness of theinterposed plain foils or sheets will be so chosen that the plain layersare capable of carrying the superposed layers substantially withoutbeing deformed so that expansion and contraction of adjacent corrugatedlayers will not be interfered with by the adjacent or contacting plainlayer, for instance, when the layers are superposed upon each other inthe manner illustrated in FIG. 8.

To proceed in this manner, i.e., by preventing the crests of thecorrugations of one layer to extend in the valleys between the crests ofcorrugations of an adjacent layer, in other words, by preventingintermeshing of adjacent corrugated layers, will not only achieve thatthe finished laminated body will be crack-free, but other advantages arealso connected therewith. Thus, the layer structure, according to thepresent invention, i.e. without intermeshing of adjacent layers, will beless dense than a structure consisting of superposed intermeshingcorrugated layers.

Consequently wider spaces will be formed between adjacent layers andthis will permit the use of more viscous impregnating resins for fillingthe spaces between adjacent layers. When using crepe-type papers ofrelatively coarse structure or corrugation, it is possible andfrequently advantageous to incorporate minera-l fillers in pulverulentform, such as quartz powder, in the hardenable synthetic impregnatingresin. This can be done successfully only if the width of the voids orfree spaces between adjacent superposed layers is relatively great. Itcould be said that, according to the present invention, the crepe or thelike layers serve primarily as resi-lent spacing and supporting elementsfor the plain, preferably smooth and frequently conductive layers whichare interposed between adjacent crepe layers. Thus, the crepe layersserve to form a reinforcing skeleton surrounded by free spaces which arerelatively large so as to facilitate filling of these free spaces withthe hardenable resin. The thus-formed laminated body consists of arelatively small proportion of layer material and a relatively largeproportion of hardened resin.

The foregoing is particularly important when a laminated body is desiredprimarily consisting of a hardened insulating resin having a pluralityof conductive, for instance metal, layers distributed therethrough. Insuch case the main purpose of the corrugated layers is to space theplain conductive layers from each other until after the impregnatingresin has been applied and hardened. The plain conductive layers shouldnot conform to the corrugations of the interposed layers of crepe paperor the like, but are only supported by the same.

For instance, in the case of condenser bushings, i.e. when it is desiredto provide an insulating covering for high voltage carrying conductivemembers of electrical apparatus and when it is further desired toincorporate conductive layers in the insulating covering, then theseconductive layers are advantageously utilized as the plain layers whichare interposed between adjacent corrugated layers, and the strength andsurface condition of the conductive layers will be so chosen as not tointerfere with contraction or expansion of adjacent corrugated layers.For this purpose the interposed conductive layers preferably will havesmooth surfaces and will be of even thickness throughout, sufiicient toprevent deformation of the conductive layer when arranged betweencorrugated or crepe paper layers. Thereby also the dielectric propertiesof the structure are improved since irregularities in the distributionof the electric field would occur if the metallic or the like conductivelayers were permitted to hug the corrugations of the insulating layers.By thus preventing irregularities in the distribution of the electricfield, localized electric over-stresses within the individual layers areavoided.

The conductive inserts or layers may consist, for instance, of aluminumfoils, aluminum foils backed by a paper layer, fine-mesh metal wirefabric, graphite paper, metallized paper or foils. Preferably, the edgeportions of such conductive layers or inserts are provided with socalledpotential rings in order to reduce the electric stress at the edges ofthe insert. In this manner, the occurrence of a high electric fieldstrength at the edges of such stress-grading layers is prevented.Aluminum foils used as plain layers interposed between crepe paperlayers may be relatively thin, for instance having a thickness of 0.01mm. If such metal foils are too thin to withstand deformation when incontact with the adjacent corrugated layers, then a paper backing or thelike should be attached to the metal foil.

Potential rings may consist, for instance, of endless metal coil springsof small coil diameter; or resilient rings with rounded edges andconductive surface may be used, such as heat resistant conductive rubberrings which, per se, are well known. The potential rings are thenconductively connected with the conductive plain layers, for instance,by welding, soldering, or adhesively with an electrically conductivelacquer. Very good results are obtained with plain conductivestress-grading layers of fine metal mesh and coil springs as describedabove serving as potential rings.

Highly advantageous results are obtained by using as corrugated layerscrepe paper which is crimped in such a manner that the possibletensioning due to crimping or creping amounts to between 10 and 50% oreven more of the original length of the crepe paper. However, it is alsopossible to use crepe papers which can be stretched to a lesser extent.

Laminated bodies of the above described type may be formed of crepepaper webs, alternating with plain webs, which are of considerablewidth, or also of narrower crepe paper bands. Thus, the respectivelayers may be formed by winding strips of the layer material about theconductive member, or also by using suitably cut sheets of the layermaterial.

As hardenable synthetic resins primarily those are to be considered,according to the present invention, which are free of solvents and whichharden inversibly without splitting off volatile constituents.

Particularly, so-called loW pressure resins such as epoxy-resins and theresins which belong to the group of unsaturated polyesters have beenfound to give good results, according to the present invention. The useof these latter resins is connected with the great advantage,particularly in the case of high voltage installations, which must befree of pores or blisters, that the laminated body can be producedwithout application of pressure andwhen cold hardening resins areapp1iedthe hardening can be carried out Without appreciable sup-ply ofheat.

Furthermore, poly condensation resins, preferably phenol formaldehydeand cresol formaldehyde resins which can be solidified by hardening andwhich can be applied in a manner known per se, can be used. When it isdesired to produce pore-free coverings with these latter resins, thenthey must be worked up under application of pressure and heat andthereby slightly prehardened resins are preferred.

Furthermore, unsaturated simple polymerizable so called monomeric oroligomeric substances may be used, such as styrene or metacrylic acidesters which are transformed into solid state by polymerization.

In addition, thermoplastic materials may be used as hardenable syntheticresins, according to the present invention, provided that thethermoplastic materials will become solidified by being cooled to theoperating temperature of the laminated body, i.e. to the highesttemperature to which the laminated body might be exposed afterinstallation. Such thermoplastic materials can be used which are ofsufficiently liquid consistency at temperatures which are still too lowto adversely affect the crimping of the crepe paper.

The following example of hardenable resins which may be used, accordingto the present invention, is given as illustrative only, the invention,however, not being limited to the specific resins mentioned therein.

Example POLYHYDRIC PHENOLS resorcinol, hydroquinone,

pyrocatechol, saligenin, phloroglicinol 4,4dihydroxy-diphenyl-dimethyl-methane 4,4 dihydroxy-biphenyl 4,4dihydroxy-diphenyl-sulfone 4,4 dihydroxy-diphenyl-methanetrihydroxy-diphenyl-dimethyl-methane polyhydric naphthalenes n-ovolacresins POLYHYDRIC ALCOHOLS glycerol ethylene glycol 2,3-butane diolerythrol sorbitol trimethylene glycol diglycerol HALOHYDRINEepichlorhydrin dichlorohydrin epibromhydrin epihalohydrins of mannitol,sorbitol, erythrol.

The preparation of such poly-epoxides is, for instance, described in thefollowing U.S. Patents: 2,324,483; 2,444,- 333; 2,467,171; 2,538,072;2,558,949; 2,582,985; 2,592,- 560; 2,665,266; 2,521,911; and in thefollowing British Patents: 746,824; 726,830.

The poly-epoxides may also be formed of compounds containing doublebands, such as: esters of oleic and linolic acid with glycerine,crosslinked pol-ybutadiene rubber (which contains terminal and internalolefin groups) epoxidized with peracetic acid.

The preparation of these compounds is, for instance, described in:Fitzgerald et a1. Epoxy-polybutadiene resins, Electronic Equipment, July1956, and in US. Patents: 2,485,160; 2,569,502; 2,458,484; 2,567,930.

These products are, for instance, available under the following tradenames:

Epon-Resin of Shell Chemical Co.; Epi-Rez of John Dabnay Co.;

Araldite of Ciba Co., Inc.;

Bakelite ERL-2774, ERL 3794, ERL-2795.

The above described poly-epoxides can be transformed into hard thermosetsolids by the following methods:

(1) by direct linkage between epoxy groups and hydroxyl groups withcatalysts such as tertiary amines and Friedel-Crafts type catalysts; forinstance as described in U.S. Patents: 2,553,718; 2,575,558.

(2) by curing with crosslinking agents:

(A) PRIMARY AND SECONDARY AMINES ethylene diamine diethylene triaminetriethylene tetramine dimethylamino propylamine piperidine metaphenylenediamine 4,4 methylene dianiline diaminodiphenylsulfone;

1 2 (B) AMIDES dicyandiamide polyamide (C) ORGANIC ACIDS oxalic acidphthalic anhydride, citraconic anhydride maleic anhydridehexahydrophthalic anhydride pyromellitic dianhydride dodecenyl succinicanhydride methylated maleic acid adduct of phthalic anhydride chlorendicanhydride maleic acid adduct 0f pthalic anhydride.

Such resin-hardener systems are, for instance, described in thefollowing US. patents: 2,324,483; 2,444,333; 2,500,600; 2,585,115;2,717,885; 2,744,845; 2,760,944; 2,773,048.

Basic condensation products may also be considered as poly-epoxy bodiesfor the purposes of the present invention.

To the above described mixtures, plastifiers, wetting agents, etc. canbe added. The preparation, use and adjustment of such epoxy-resins hasbeen described in Henry Lees Epoxy-Resins, published by McGraw-Hill BookCompany, Inc., 1957.

Unsaturated polyester-type resins are, for instance, described in thefollowing publications: C. P. Vale: The chemistry of unsaturatedpolyester resins. British Plastics, September 1953, pp. 327-332, US.Patent 2,604,463.

They are formed by copolymerization of unsaturated polyesters, fromglycols and unsaturated dibasic acids with vinyl type monomers such asstyrene, diallylphthalate, triallycyanurate, etc. in the presence of acatalyst such as benzoyl peroxide, methyl-ethyl-ketone peroxide,cyclohexanone peroxide etc.

While the laminations, according to the present invention, may be formedin many different ways, it is particularly advantageous to form the sameby winding the plain and crepe paper webs or bands about a conductor orthe like, or about a temporary mandrel. Thereby care has to be taken towind as tightly as possible. Furthermore, the crepe paper should not betensioned too strongly during the winding so that under allcircumstances the Wound crepe paper retains at least part of itscrimping. Thereby, those crepe paper types which, due to their crimping,can be stretched by more than of their original length can be tensionedmore strongly than crepe paper types of lesser crimping. Furthermore,the crepe paper is to have such mechanical strength as to allow theforming of a dense roll without excessive stretching of the paper.

When it is desired to provide laminated coverings for straight memberssuch as cylindrical members or rods, or also on multi-edged profiles, itis frequently advantageous to use, according to the present invention,plain and crepe paper webs having a width corresponding to the width ofthe body which is to be covered.

It is important, of course, that the individual corrugated layers andplain layers are so superposed upon each other that independentexpansion and contraction of the individual corrugated layers orportions thereof remains possible. Thus, when a cylindrical conductivemember is to be provided with a laminated covering, individual tubularlayers of corrugated sheet material and of plain sheet or foil materialhaving progressively increasing diameters may be prepared and may thenbe sequentially slipped over the cylindrical conductor and thepreviously applied layers. Here again, the individual corrugated tubularlayers preferably will be pretensioned. Some or all of the plain layersmay consist completely or partially of conductive material so as to formstress-grading layers.

The supporting plain layers must not be adhesively or otherwise adheredto the adjacent corrugated layers such as crepe paper layers, since suchadherence would prevent the independent expansion or contraction of thecorrugated layer which should be possible in order to equalize thedegree of pretension of the corrugated layer and in order to permitadjustment of the corrugated layer to any dimensional changes occurringin the impregnating resin during hardening of the same.

When it is desired to form a crack-free laminated covering on a verylong body or ona body having a curved axis, then preferably relativelysmall bands are helically wound about such body, for instance a highvoltage conductor, so as to form alternating coaxial corrugated andplain layers. In the case of curved conductors, the bands may be so cut,considering the pitch of the helical winding, the radius of thecurvature of the conductor and the diameter of the same, thatoverlapping of band portions is avoided.

The corrugated layers, particularly if the same are of a coarsestructure so that the thickness measured between the crests of oppositecorrugations equals 2 mm. or more, as well as the interposed plainlayers may be formed with perforations or cutouts therein which willpermit passage of impregnating resin in radial direction therethrough.

Referring now again to the drawings, and particularly to FIG. 6, thesame shows a portion of a sheet of such corrugated or crepe paper ofcoarse structure.

FIG. 7 illustrates a crepe-type or pleated paper which may be stretchedto a very high degree and which possesses a much more even and symmetricstructure than the sheet of FIG. 6. This type of paper, particularlywith respect to resilient tensioning thereof, shows a behavior which issomewhat similar to that of conventional crepe paper and thus permitsapplication while being in partially pretensioned state, namely so thatthe creping or pleating is partially retained when the sheet or band iswound about the conductor or the underlying plain layer. This type ofpleated paper is commercially available, for instance, under the tradename Carbion from the firm Baumgartner of Lausanne, Switzerland.

According to FIG. 8, crepe paper bands, or bands of other types ofpreferably pretensioned corrugated material are wound about an electricconductor 11. Thereby, on top of each completed wound layer of crepepaper such as layers 12, 13 and 14, a plain supporting intermediatelayer, such as layers 15 and 16, is wound followed by another crepepaper layer. Layers 15 and 16 may be conductive layers, consisting forinstance of metal foil, or layers 15 and 16 may be plain paper layerswhich also may serve as support for a further (not illustrated)intermediate electrically conductive layer.

The intermediate plain layers may be superposed upon the underlyinglayer of crepe paper or the like in the form of a wide web such as layer15, or the intermediate layer may be formed by helically winding arelatively narrow band 16.

The skeleton of the laminated covering which is formed according to FIG.8 about a straight cylindrical conductor, may also be arranged about acurved conductor, in which case the layers preferably are formed byhelical winding of relatively narrow bands. However, when a corrugatedpaper of relatively coarse structure is to be wound, then such band ispreferably pre-cut so as to avoid overlapping at the inner periphery ofthe curved conductor.

Particularly good results are obtained when as synthetic resin forimpregnating the skeleton structure of alternating corrugated and plainlayers, a hardenable casting resin such as an epoxy resin or anunsaturated polyester resin is used in which preferably a pulverulentfiller material such as quartz powder has been incorpo-- rated.

In addition to the above described corrugated papers,

it is sometimes also desirable to use embossed papers or embossed crepepapers which possess the added advantage of forming in superposedrelationship larger free spaces, corresponding to the embossing of thepaper, whereby the use of more viscous resins or pulverulentfiller-containing resins is facilitated. Embossed crepe papers suitablefor this purpose are commercially available, for instance, asPerlkrepp-Papers.

The plain intermediate layers may also be formed wholly or in part ofsuch material that they will not only carry or support the superposedcorrugated layer without being deformed but in addition will reinforceand give additional mechanical strength to the entire laminatedstructure. Thus, the plain layer may consist of various types offabrics, such as glass fiber fabrics or fabrics of synthetic materials,fleeces, or foils. Such reinforcing intermediate plain layers may alsoconsist of composite sheets, the reinforcing fabric, for instance,forming a backing for a metal foil thereto.

The last described embodiments of the present invention will be furtherillustrated in the following examples without limiting the invention tothe specific details of the examples.

Example 4 A laminated insulating covering is to be formed about acylindrical copper rod having a straight axis, a diameter of 30 mm. anda length of 1500 mm., for instance, a condenser bushing. The laminatedcovering is to include five layers of a crepe paper or pleated papersuch as is illustrated in FIG. 7. Between adjacent crepe paper layers aplain, smooth conductive layer is to be arranged. The length of thesuperposed conductive layers is reduced in the direction from theinnermost to the outermost layer, as is customarily the case incondenser bushings. The crepe paper layers extend laterally at bothsides respectively about 5 mm. beyond the next superposed conductivelayer.

The crepe paper, in creped condition, weighs 330 grams per square meter,has an extensibility of 300% and a thickness in creped condition of 3.8mm., while the thickness of the same but uncreped paper equals 0.2 mm.

The crepe paper is cut into rectangular pieces, the length of whichexceeds the length of the sheets of the condutcive layer material by 10mm. The width is equal to of the calculated circumference of therespective tubular layer which is to be formed of the sheet, plus 5 mm.,so that the sheet will be stretched when wound about the copper rod. Thethus-cut pieces of crepe paper are formed into tubes with thecorrugations extending coaxially with the thus-formed tubes to allowupon stretching to increase the tube diameter. The width of the sheetwill now form the circumference of the tube, however, 5 mm. of the widthare arranged overlapping each other so that for 2 /2 mm. of thecircumference and extending longitudinally there will be an intermeshingof the superposed end portions of the cut, now tubular sheet.

The thus-formed tubes are coaxially superposed upon each other and uponthe copper rod. A tubular sheet of smooth graphited kraft paper having athickness of 0.15 mm. and weighing 113 grams per square meter isarranged between each pair of adjacent tubular crepe paper layers sothat contact between adjacent crepe paper layers is prevented. Duringsuperposing of the individual crepe paper layers the same are stretchedby about 50%. The total wall thickness of the composite structurecomprising 5 tubular crepe paper layers and 4 interposed conductivelayers equals about 19.5 mm.

The thus-covered conductor is now placed into a cylindrical mold havingan inner diameter of 75 mm. and fixed therein coaxially with the mold.The mold has a closed bottom and a funnel for introducing resin at itsupper end.

The mold holding the conductor with the crepe paper and conductivelayers about the same, is now placed into a heatable vacuum containerand is therein dried and degassed for 6 hours at a temperature 80 C. andat a residual pressure of 0.5 mm. Hg. Thereafter, the casting resin isintroduced through the funnel into the mold while vacuum and atemperature of 80 C. are maintained as before. Thirty minutes afterintroduction of the resin atmospheric pressure is restored, and theresin is allowed to harden for 12 hours at 90 C. and thereafter for 8hours at 120 C. Thereafter the mold and its contents are allowed to coolto ambient temperature and then the thus-formed insulated condenserbushing is removed from the mold. A ground connection will be eventuallyattached to the outermost conductive layer.

The casting resin mixture is formed of 100 parts by weight of epoxyresin on the basis of p,p-dioxydiphenylpropane having an epoxyequivalent weight of between 185 and 200 and a viscosity of between11,000 and 14,- 000 centipoises at 25 C.; plus 130 parts by weight ofdodecenylsuccinic acid anhydride; plus 0.2 part by weight ofmethylbencyldimethylamine; plus 350 parts by weight of quartz powder of325 mesh per inch (according to Tylor).

Example A straight cylindrical copper rod of 40 mm. diameter and 1000mm. length is surrounded, as described in Example 4, with alternatintubular crepe paper layers and tubular plain sheet layers, the crepepaper layers initially serving as support and spacing elements.

The crepe paper, in creped condition, has a weight of 250 grams persquare meter, is capable of being stretched by 30%, has a thickness of0.8 mm. in creped condition, while the thickness of the sheet prior tocreping equals 0.2 mm.

The crepe paper is cut into bands of 30 mm. width and helically woundabout the copper conductor. During such winding, the crepe paper isstretched by 7%. After forming in this matter a first layer of crepepaper on the copper conductor, a single sheet of smooth kraft paperhaving a thickness of 0.15 mm. and a weight of 114 grams per squaremeter is wound about it. The rectangular sheet of kraft paper has beencut to the length and circumference of the conductor with the firstcrepe paper layer thereon, so that the sheet covers the entire freesurface of the first crepe paper layer. Thereafter, the next crepe paperlayer is wound about the last formed layer of kraft paper, and this isfollowed by helically winding a band of 15 mm. width of cable paper fora desired length about the previously formed layer. The cable paperconsists of a paper backing having a thickness of 0.15 mm. and havingadhered thereto an aluminum foil of 0.008 mm. thickness. Foil andbacking paper are formed with perforations of 1 mm. diameter. The lengthfor which this conductive layer covers the copper rod depends on thedesired distribution of the electric field.

Further layers are applied until a total of 18 crepe paper withinterposed kraft paper layers with 9 interposed conductive layerssurrounds the copper rod.

The thus-covered conductor is then installed in a cylindrical mold,dried and heated and then impregnated with a resin mixture, as describedin Example 4. The resin mixture has the following composition: 100 partsby weight epoxy resin as described in Example 4; plus 130 parts byweight of d-odecenylsuccinic acid anhydride; plus 0.2 part by weight ofmethylbencyldimethylamine; plus 100 parts by weight of ground dolomitehaving a particle size of 20 microns.

Example 6 A copper rod of mm. diameter and a total length of 1600 mm. isbent in its middle portion to an angle of 45 with a radius of 600 mm.Strips of crepe paper such as described in Example 4 with thecorrugations thereof extending in transverse direction and having aWidth of 40 mm. are helically wound about the copper rod so that in thestraight portion of the rod the edges of adjacent windings contact eachother. In the bent portion, the crepe paper band is so cut as to preventsubstantial overlapping of adjacent windings along the inner peripheryof the copper rod. During the winding of the crepe paper the same isstretched by 50% of its original length. Upon each thus-formed layer ofcrepe paper, a layer of cable paper of the type described in Example 5is wound in the form of a band of 15 mm. widths. Again, the portion ofthe length of the copper rod along which the conductive layers areformed depends on the desired distribution of the electric field.

A total of 4 crepe paper layers with respectively superposed conductivelayers are wound about the copper rod and the length of the individualcrepe paper layers is adjusted corresponding to that of the adjacentconductive layers. The rod with the layers wound about the same is nowinstalled in a curved mold of 70 mm. inner diameter coaxially therewithi.e. in such a manner that its distance from the inner mold wall will bethe same throughout. The mold is closed at its lower end and providedwith a funnel at its upper end. The mold is then installed in as uprighta position as possible in a heatable vacuum container, dried anddegassed for 12 hours at 40 C. under a residual pressure of 0.5 mm. Hg.Thereafter, the resin mixture is allowed to flow into the mold at atemperature of 40 C. while vacuum is maintained. Thirty minutes afterintroduction of the resin atmospheric pressure is restored and the resinis then allowed to harden first for 8 hours at C. and then for 12 hoursat C. l

The resin mixture consists of 100 parts by weight of Rhodester 1108 R asolution of an unsaturated polyester in styrene made by the firmRone-Poulenc of Paris, France, and having a viscosity of 300 centipoisesat 25 C.; plus 1 part by weight of bencylperoxide; plus parts by weightof quartz powder, 325 mesh.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types oflaminated coverings differing from the types described above.

While the invention has been illustrated and described as embodied in aninsulated electrically conductive body, it is not intended to be limitedto the details shown, since various modifications and structural changesmay be made without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. In a method of producing an electrical apparatus, the steps ofarranging about an elongated electrically conductive member coaxiallytherewith a plurality of superposed continuous first layers ofinsulating material and of continuous second layers alternating withsaid first layers, said first layers, respectively, being formed with aplurality of spaced projections extending toward and contacting adjacentones of said second layers that adjacent convolutions of said secondlayers are completely separated from each other by the interveningconvolutions of said first layers and intermeshing of adjacentconvolutions of said first layers is prevented and, due to the spacedprojections of said first layers, the intervening spaces betweenadjacent convolutions of said second layers and adjacent said conductivemember are divided into a plurality of passages; subjecting the thusformed structure to a partial vacuum so as to subject the same to dryingand degassing; filling said passages while maintaining said partialvacuum with a hardenable casting resin in fiowable state; and allowingsaid casting resin filling said passages to harden, whereby a unitary,insulated portion of an electrical apparatus is formed.

2. A method of forming a rigid crack-free laminated covering on a solidbody, comprising the steps of elastically stretching corrugated sheetmaterial havinga predetermined maximum extensibility, said stretchingbeing carried out so as to stretch said corrugated sheet material onlyto a fraction of its maximum extensibility; alternatingly applying to atleast a portion of the surface of a solid body layers of saidelastically stretched corrugated sheet material and layers of asubstantially plain sheet material of sufficient strength to remainsubstantially unafiected by adjacent layers of said stretched corrugatedsheet material, so as to form a composite structure of alternatinglyarranged superposed layers of said stretched and thus tensionedcorrugu-ated sheet material and of said substantially plain sheetmaterial in such a manner that contraction and expansion of saidtensioned corrugated sheet material is unobstructed by said alternatinglayers of substantially plain sheet material and so that adjacentconvolutions of said corrugated sheet material are completely separatedfrom each other by intervening convolutions of said plain sheet materialand intermeshing of ad jacent convolutions of said corrugated sheetmaterial is prevented and, due to the corrugations of said corrugatedsheet material, the intervening spaces between adjacent convolutions ofsaid plain sheet material and adjacent said solid body are divided intoa plurality of passages; subjecting the thus formed composite structureto a partial vacuum so as to cause drying and degassing thereof;impregnating said composite structure while maintaining said partialvacuum with a synthetic hardenable resin so as to fill all voids Withinsaid composite structure with said hardenable resin; and hardening saidresin whereby due to the tensioned state of said corrugated sheetmaterial, the same will conform to any dimensional changes caused by thehardening of said resin thereby forming a rigid crack-free laminatedcovering consisting essentially of said superposed layers of tensionedcorrugated sheet material and layers of said plain sheet materialadhered to each other by said hardened synthetic resin filling thepassages therebetween.

3. A method of forming a rigid crack-free laminated covering on a solidbody, comprising the steps of elastically stretching crepe paper havinga predetermined maximum extensibility, said stretching being carried outso as to stretch said crepe paper only to a fraction of its maximumextensibility; alternatingly applying to at least a portion of thesurface of a solid body layers of said elastically stretched crepe paperand layers of a substantially plain sheet material of sufficientstrength to remain substantially unaffected by adjacent layers of saidstretched crepe paper, so as to form a composite structure ofalternatingly arranged superposed layers of said stretched and thustensioned crepe paper and of said substantially plain sheet material insuch a manner that contraction and expansion of said tensioned crepepaper is unobstructed by said alternating layers of substantially plainsheet material and so that adjacent convolutions of said crepe paper arecompletely'separated from each other by intervening convolutions of saidplain sheet material, and inter-meshing of adjacent convolutions of saidcrepe paper is prevented and, due to the projections of said crepepaper, the intervening spaces between adjacent convolutions of saidplain sheet material and adjacent said solid body are divided into aplurality of passages; subjecting the thus formed composite structure toa partial vacuum so as to cause drying and degassing thereof;impregnating said composite structure while maintaining said partialvacuum with a synthetic hardenable resin so as to fill all voids Withinsaid composite structure with said hardenable resin; and hardening saidresin whereby due to the tensioned state of said crepe paper, the samewill conform to any dimensional changes caused by the hardening of saidresin thereby forming a rigid crack-free laminated covering consistingessentially of said superposed layers of tensioned crepe paper andlayers of said plain sheet material adhered to each other by saidhardened synthetic resin filling the passages therebetween.

4. A method of forming a rigid crack-free laminated covering on a solidbody, comprising the steps of elastically stretching corrugated sheetmaterial having a predetermined maximum extensibility, said stretchingbeing carried out so as to stretch said corrugated sheet material onlyto a fraction of its maximum extensibility; alternatingly applying to atleast a portion of the surface of a solid body layers of saidelastically stretched corrugated sheet material and layers of asubstantially plain sheet material of sufiicient strength to remainsubstantially unafiected by adjacent layers of said stretched corrugatedsheet material, so as to form a composite structure of alternatinglyarranged superposed layers of said stretched and thus tensionedcorrugated sheet material and of said substantially plain sheet materialin such a manner that construction and expansion of said tensionedcorrugated sheet material is unobstructed by said alternating layers ofsubstantially plain sheet material and so that adjacent convolutions ofsaid corrugated sheet material are completely separated from each otherby intervening convolutions of said plain sheet material, andintermeshing of adjacent convolutions of said corrugated sheet materialis prevented and, due to the corrugations of said corrugated sheetmaterial, the intervening spaces between adjacent convolutions of saidplain sheet material and adjacent said solid body are divided into aplurality of passages; subjecting the thus formed composite structure toa partial vacuum so as to cause drying and degassing thereof;impregnating said composite structure while maintaining said partialvacuum with a synthetic hardenable resin having quartz powderdistributed therethrough so as to fill all voids within said compositestructure with said hardenable resin; and hardening said resin wherebydue to the tensioned state of said corrugated sheet material, the sameWill conform to any dimensional changes caused by the hardening of saidresin thereby forming a rigid crackfree laminated covering consistingessentially of said superposed layers of tensioned corrugated sheetmaterial and layers of said plain sheet material adhered to each otherby said hardened synthetic resin filling the passages therebetween.

5. A method of forming a rigid crack-free laminated covering on anelongated substantially cylindrical solid body of predetermineddiameter, comprising the steps of forming a plurality of first tubes ofcorrugated sheet material the corrugations of' which extendsubstantially parallel to the axes of said tubes, said tubes being ofprogressively increasing diameters so that said tubes may be insertedinto each other in spaced relationship, the diameter of the smallesttube being sufiicient to permit in sertion of said substantiallycylindrical solid body; forming a plurality of second tubes of plainsheet material of progressively increasing diameters such that saidtubes of plain sheet material may be inserted between adjacent ones ofsaid corrugated tubes; assembling said tubes about said cylindricalsolid body so as to form a composite cylindrical structure formed ofsaid cylindrical solid body surrounded by alternatingly arranged tubularlayers of corrugated and of plain sheet material; subjecting the thusformed composite structure to a partial vacuum so as to cause drying anddegassing thereof; impregnating the thus formed composite structureWhile maintaining said partial 19 20 vacuum with a synthetic hardenableresin so as to fill all 3,112,357 11/1963 Imhof 174-121 voids therein;and hardening said resin thereby forming 3,127,470 3/1964 Anderson et aLa rigid crack-free laminated covering on said elongated 3 265 799 8/1966Mcwhirter solid body.

References Cited 5 AR M B R E T P UNITED STATES PATENTS E L E G R nmaryExwmmer.

2,224,810 12/1940 Cumfer 156195 X T. R. SAVOIE, Assistant Examiner.

3,025,340 3/1962 Olson 174120

